This invention is specifically directed to an improvement of the cartridge, and module in which the cartridge is used, both disclosed in our parent application. "Cartridge" refers to an assembly of "wafers" which assembly is constructed by sequentially assembling frames and monolayer arrays, seriatim. We sought to form a cartridge which could be "ganged" with other cartridges to form a "stack" in a module, using appropriate gasket means for disassembly of the cartridges, if disassembly was desired. The parent cartridge was assembled with plural unitary wafers each having a continuous, imperforate border. Since a serviceable cartridge is expected to provide "zero defect" service over its expected life time, the cost of maintaining "zero-tolerances" required for constructing each wafer as it was incorporated into the cartridge, becomes difficult to pass on to the end user of a module in which the cartridges are used.
This invention is directed to a cartridge of more economical construction, made possible because it is less demanding as to tolerances without sacrificing "zero defect" service. Such service is made possible because the cartridge is made with wafers, each having a continuous, but perforate or "slotted" border. The term "wafer", and more specifically "slotted wafer", is used in the invention claimed herein, to refer to an array of a single layer of laterally spaced-apart fibers secured in a frame bounded by a continuous border having at least two opposed elongate through-passages or "slots" within opposed portions of the border. Opposed terminal end portions of the fibers are held in the opposed, now "slotted" border portions. The cartridge comprises at least three, typically from 10 to 200, that is, a multiplicity of unitary wafers assembled seriatim, one in congruent contact with another, in a cartridge. The vertically aligned slots of successive wafers, the ends of which slots are in open fluid communication with each other in a cartridge of wafers, are potted after the cartridge is constructed. The vertically aligned ends of successive slots, and the slots themselves, together function as a "potting channel". Because the potting channel is "filled with resin" after the cartridge is assembled, the cartridge is referred to as being "post-potted". When post-potted, a fluid-impermeable continuous annular shell of potting resin is formed within the cartridge, the annular shell surrounding the fibers.
The term "fiber" is used herein, for brevity, to refer, both, hollow fiber organic polymeric membranes, and also to hollow fiber ceramic/metallic (ceramic, or metallic, or both) membranes, except when the latter is specifically referred to. Under such a circumstance, because ceramic/metallic membranes are typically much larger in diameter than organic polymeric fibers, the ceramic/metallic hollow fiber membranes are also referred to as "hollow tube membranes".
Though numerous embodiments of framed hollow fiber membranes (referred to as "fibers" for brevity) have been disclosed in this art, and much effort has been expended to provide a "wafer" or "cell" which can be vertically assembled, one in direct contact with the other, and upon it (referred to herein as being "stacked" to form a module), the effort has not resulted in a sufficiently cost-effective module which is reliable, rugged and has wide commercial applications. The term "cell" is used to refer, in addition to an array, in a more general sense, to a prior art assembly of fibers within a frame.
It will be noted that each wafer is not a cell since an individual wafer has no meaningful existence. The wafers come into existence only when stacked to form a cartridge. Unlike in the prior art, an assembly of stacked wafers cannot be used until the uppermost array of fibers is covered with an end cap. In other words, the minimum "unit" of this invention is neither a cell nor a wafer but a cartridge.
As will be explained in greater detail herebelow, the construction of a cartridge of slotted wafers was not taught in the prior art for a number of inter-related reasons, not the least of which was that the art suggested neither why nor how such a slotted cartridge could be constructed, nor did the art intimate the benefits of such construction.
In the particular instance when the membrane is in the form of a capillary tube or hollow fiber, and used for filtration, the membrane-material divides the module into a "feed zone" and a "non-feed zone", the latter referred to as the "permeate zone" regardless of whether the module is used in a filtration application. The feed which is introduced either externally (referred to as "outside-in" filtration) or internally ("inside-out" filtration) of the fibers, is resolved into "permeate" and "concentrate" streams. Many physical considerations affect the operation of a module. For example, the permeability and rejection properties of the membrane; the process flow conditions such as pressure, rate of flow, temperature, etc.; the physical and chemical properties of the feed and its components; the relative directions of flow of feed and permeate; the thoroughness of contact of the feed with the walls of the fibers; and other parameters, each has a direct effect on the efficiency of the module. The goal is to maximize the efficiency of the module and to do so in a practical and economic manner.
Readily apparent is that channelling of the feed due to uneven distribution of the fibers will result in substantially poorer performance of the module than if the feed flowed evenly over the surface of each fiber in the bundles of fibers. Since the rate of transfer of the desired component of the feed from one side of the membrane to the other is necessarily relatively slow, to obtain an optimum rate of transfer requires maximizing the surface area of the membrane and maintaining an effective driving force such as a concentration or pressure differential between opposed surfaces of the fibrous membrane. As a result it becomes essential to use a multiplicity of long fibers of small diameter within the casing (shell) of a module so as to minimize the void (unoccupied space) therewithin without packing the fibers together too tightly, and to use as high a fluid velocity or pressure differential as the physical and economic circumstances will permit. But long fibers are susceptible to damage, the longer the fiber, the greater the susceptibility.
If the fibers are packed too tightly in "outside-in" filtration, the throughput of feed cannot be increased in proportion to the number of fibers used because of the increased pressure drop. If the length of fibers is increased too much the resistance of the flow path within the bores of the fibers becomes a limiting factor which limits the throughput of feed, though the pressure drop through the bundle of fibers in the module may not be a factor.
In the particular instance of filtration, using such "outside-in" flow of feed, not only does the feed flow through the path of least resistance, namely the largest voids, but it will also tend to collect in some voids from which flow is impeded. The result is that the concentration of a residual component ("residue") remaining on the outside of the fibers ("concentration polarization"), increases. Some of the residue will pass through the walls of the fibers and increase the concentration of the residue impurity in the permeate. If the residue is a salt, some, when concentrated will precipitate on the fibers' surfaces reducing their effective area available for permeation, a phenomenon known as "fouling".
To counter the problems of poor flow of feed through a module and the resulting inefficient mass transfer of the permeate across the membrane, numerous schemes have been suggested, some more practical than others. All are characterized by a conspicuous absence of details as to packing efficiency of the fibers in the module, and most particularly, how one might "fix" the orientation of the large number of fibers required in a practical module.
Routinely, fibers are "potted" near one, or near opposed ends, in a hardened synthetic resinous "header" which is adapted to be inserted with suitable gasketing means, in fluid-tight engagement with the interior wall of the casing of the module. A conventional potting procedure is to insert the terminal portions of a multiplicity of vertically oriented fibers in laterally slightly spaced-apart relationship with one another, with a first set of one ends of each of the fibers pointing down, into an appropriately shaped container of potting resin (approximately the shape of a header). The inner walls of the container are coated with a release agent before the resin is poured into the container. The first set of one (first) ends of the fibers are held in position until the resin cures and a header of cured resin is removed. The end surface of this first header is then cut off exposing the open ends of the fibers in the face of the header. The face of this first header then appears foraminous because of the planarly disposed, exposed ends of the fibers, the solid resin forming a seal around the exterior terminal portions of each fiber.
The same procedure is then repeated to form a second header with the other (second) set of the other ends of the bundle of fibers. The headers may then be secured in fluid-tight engagement within each end of a shell to form a module.
Another option is to pot both sets of ends of the fibers in the same header by wrapping the fibers around one end of a frame having elevated elongated sides, as was done by Baudet et al in U.S. Pat. No. 3,993,816. With this arrangement one hopes to gain more from packing a large number of fibers into each frame, by curing one layer of fibers, then placing another (second) layer in staggered relationship with the fibers of the preceding layer, and curing the second layer, than one sacrifices in the non-productive bends of the fibers.
In addition to coping with the problem of positioning a large number of fibers precisely before they are potted, there are numerous pitfalls in conventionally "potting" the terminal portions of fibers in a resin which can be solidified. To begin with, one must find a resin which is sufficiently compatible with the fibers as to form a fluid-tight bond which will survive over the useful life of the module. After having found such a resin one must make sure that movement of the fibers near the resin does not damage the fibers due to the shearing action of the solid resin on their terminal portions, particularly if the pressure differential to be used in the module is substantial. Further, cutting and dressing the solid resin to expose the ends of the fibers may result in plugging many of the fibers.
For example, Nichols in U.S. Pat. No. 4,959,152, states "Hollow fiber membranes may be conveniently mounted in annular or similar frames or retainers having a continuous perimeter and an open central portion. The fibers are strung across the open central portion of the frame and the ends are embedded in the retainer thereby forming a wafer. The ends of the fibers are exposed at the outside surface of the retainer, giving access to the interior of the fibers, while the outside surfaces of the fibers are accessible in the open central portion of the retainer." (see col 1, lines 57-66). Soon thereafter he states "Tight sealing of adjacent wafers is essential to avoid contamination of retentate and permeate." Though not explicitly stated, Nichols recognized the importance of sealing plural layers of fibers in each wafer effectively, because he constructed a device to centrifuge an epoxy resin of appropriately chosen viscosity and quick-setting characteristics, to generate a potting ring through which the ends of the fibers protrude to discharge fluid flowing through the lumens of the fibers.
The effectiveness of the centrifugal force however was not restricted to ejecting the epoxy resin radially outward to be deposited against the inner periphery of his mold; the centrifugal force also displaced the fibers in each layer resulting in uncontrolled spacing of fibers and "gaps" which invite channeling. To counter such displacement, individual fibers running parallel to each other in the weft direction in each layer, were woven together ("tied") with warp filaments to form a flat sheet; or, each layer of fibers was adhesively secured to a contiguous layer with a suitable adhesive coated filament placed on the upper and lower surfaces, respectively, or both, of each layer (see col 4, lines 56-66). Tying fibers together results in chafing at the "ties" and premature rupture of the chafed fiber; and, in entrapment of solids in a "cage" formed by the axial zone between tied fibers.
Since our concern was to construct a wafer carrying but a single layer ("monolayer") of parallel, spaced-apart fibers without tying them together or interconnecting them, but securing them to a frame in fluid-tight spaced-apart relationship at their terminal portions, the problem of confining those terminal portions had little in common with the problem so recently solved by Nichols, namely of securing multiple layers by forming inner and outer potting rings of centrifuged resin, and removing the outer one.
More than a score of years earlier, Strand in U.S. Pat. No. 3,342,729 had to use a mesh of fibers which he sandwiched between two extruded or cast frame members, formed from a suitable thermoplastic polymeric material. The reason he was forced to use a mesh was because such a configuration of meshed fibers had inherent stability, one fiber reinforcing another in the same layer or cell of fibers. A multiplicity of individual, loose fibers, if held only near their opposed terminal end portions, do not have such stability. The stability afforded by the mesh is sufficient to allow the fibers (as a portion of mesh) to be handled and positioned between the frame members. Strand did not suggest positioning individual fibers, in side-by-side relationship between the frame members nor could he have done so without envisioning the possibility of providing an essentially planar array of fibers between frame members. The fibers as a mesh, when sandwiched between two frame members, is referred to as a "cell" in Strand's invention. By "array" we refer to a multiplicity of substantially linear fibers, individually secured in laterally spaced-apart coplanar relationship on the border, without stabilizing the fibers by interconnecting them to one another.
Strand suggested making a cell as follows: "A mesh membrane can be sandwiched between two such (frame) members and the assembly subjected to heat sealing conditions whereby a unitary, integral cell member is provided. This means has the added feature of readily and securely bonding the members into an intimate joined relationship, but additionally avoids the need for any adhesive and sealant material and the attendant setting or drying time. Means can also be provided to simultaneously heat-seal the ends of any fibers protruding beyond the outer edge of the joinder of the two frames by causing the material of the frame to flow over the joinder forming a smooth surfaced seamless edge. Care must be exercised that the fibers are not materially altered in any portion where flow therethrough is desired. The frames can be made in pairs with mating male and female fittings such as lugs and indents to facilitate and assure alignment of the various matching openings. Rapid production of the cells can be achieved by the foregoing means." (see col 7, lines 12-30).
Strand's only description of the actual technique he used to form a cell required that the sides of a frame be coated with adhesive such as an epoxy resin, and the mesh (which is cut slightly larger than the frame) be sandwiched in the adhesive between the frame members until the mesh is securely and permanently bound to the frame members. Irrespective of how the mesh is held between frame members, the fibers chafe one another at the contact points when the module is used (placed in service), and, sooner rather than later, the chafed fibers fail.
Because Strand provided peripheral flow channels in the sides of the frame, which channels could not be in fluid communication with the central opening of the frame, it was essential that there be no leakage of fluid through openings or channels in a "sealed zone" of any frame. The sealed zone of a frame is defined as the space between opposed frame members, which space is to be filled with fibers which are sealingly secured in adhesive around the fibers.
The problem we addressed was quite different from the one addressed by Strand. We sought to form a cartridge of stacked wafers, each wafer consisting of an array of a single layer of substantially coplanar, non-displaceable, individual, essentially linear fibers; supported in a substantially coplanar unitary laminar frame having a continuous but perforate or "slotted" border.
The vertical spacing between the frames of essentially contiguous wafers in a cartridge is most preferably, insignificant, being only the thickness of adhesive, if an adhesive is used; and, being none (zero vertical spacing) when the lateral surfaces of frames of a cartridge are solvent-bonded, or bonded with ultrasonic waves, or the like. The vertical spacing between next-adjacent wafers is significant when successive wafers are separated by a gasket means, the spacing being the thickness of the gasket. In all cases, the fibers of successive wafers are in vertically spaced-apart relationship with each other, the magnitude of the spacing depending upon the bonding means used to bond successive arrays in a cartridge.
In the wafer we sought to construct, the fibers were also to be in laterally spaced-apart relationship without being secured to one another intermediate their terminal portions, either to adjacent fibers in a specific array, or to adjacent fibers in an array above or below the specific array. Fibers in contact with each other not only decrease the effective area of a module of multiple wafers, but also exhibit a proclivity to chafe against each-other, as stated hereinabove. Still further, we sought to provide a post-potted cartridge of multiple wafers, each with a unitary frame preferably distinct from the adhesive which secures the fibers to the frame, and to avoid the problems and cost of machining one centrifuged (outer) layer of resin in which the ends of the fibers are plugged, to expose another centrifuged (inner) layer of resin in which the ends of the fibers are not plugged, as in the Nichols cell.
The significance and importance of securing loose, individual linear fibers in each array of coplanar fibers having generally parallel longitudinal axes, is better appreciated by referring to numerous prior art cells in which fibers are looped about a frame before their ends are secured by potting them. Even before Strand's invention teaching opposed headers in the periphery of each cell, Lewis et al in U.S. Pat. No. 3,198,335 taught a cell in which fibers were also secured in a "header" of the cell, in loops or "hanks", rather than individually, and at least one end of each loop was secured by being potted in resin to form the header. (see col 6, lines 16-30). The desirability of using loops in a cell construction having a header built into the cell, was reinforced twenty years later in an improvement by Ostertag in U.S. Pat. No. 4,440,641. In the construction of such cells, the fibers must be looped because there is no other means for holding the fibers in place before they are potted, and it is self-evident that the fibers must be held in place before they are potted.
In addition to coping with the problem of positioning a large number of fibers precisely before they are potted, there are numerous pitfalls in "potting" the terminal portions of fibers in a fluid resin which is to be solidified. To begin with, one must find a resin which is sufficiently compatible with the fibers as to form a fluid-tight bond which will survive over the useful life of the module. After having found such a resin, one must make sure that movement of the fibers near the resin does not damage the fibers due to the shearing action of the solid resin on their terminal portions, particularly if the pressure differential between the feed zone and the permeate zone in the module during its use, is high enough to cause damage. Further, cutting and dressing the solid resin to expose the ends of the fibers may result in plugging many of the fibers, and is to be avoided.
As if these problems were not enough, one has to cope with the geometry of the frame, the lateral surfaces of which are to support each array of fibers, whatever the array's configuration, in a cartridge of wafers, or a stack of cartridges, to be housed in a module. This required development of a technique (a novel method) for securing the terminal portions to the border of the frame in such a manner as both, to facilitate potting of those terminal portions of the fibers, and also to provide adequate support only at the terminal portions, to enable them to be bonded to the frame.
It will be appreciated that, though the description of the invention herein is for "outside-in" filtration of feed, the fiber array and a module containing a cartridge of arrays, or a stack, may be equally well adapted for "inside-out" filtration, for those process considerations which demand such flow.
The module containing a cartridge or stack of our invention, the method of constructing the wafer, the cartridge, and the stack, and the effectiveness of each of the foregoing in a variety of permeation processes, address the deficiencies of the prior art.